The present invention relates to an alignment apparatus for aligning an air data sensing probe and a method of using the apparatus when the probe is in place on a previously leveled aircraft. A tool is installed on the probe base plate in a known position relative to the probe axis. The tool carries an angle indicator to provide an indication when the probe axis is properly aligned relative to a horizontal or level axis. In one form, an elongated support on the tool has indicators to establish a reference line that is leveled using an external reference, such as a laser beam or an optical line. A second form of the tool has a digital inclinometer installed thereon for directly indicating angular position. The probe and the apparatus are rotated to a position where the indicator is at the desired position, and the probe is then at a known angular position relative to the aircraft axis for measuring angle of attack of the aircraft.
When using a multifunction air data sensing probe which measures pitot and static pressures and Angle of Attack, it is critical to obtain the proper alignment or relationship of the probe axis and the aircraft longitudinal plane. The mounting of the probe must be consistent for repeatability of angle of attack measurements and also should allow interchangeability of probes to reduce maintenance efforts.
It has been known to use machined dowel pins that fit closely into bores for indexing parts on the aircraft body. The dowel pins located and installed during the alignment procedure can be left as the aircraft wall. When a probe is removed and replaced, or when a different probe is replaced onto the aircraft, it is located with the dowel pins on the aircraft wall or plate placed in the bores on the probe base plate, and the probe is then at a known aligned position.
However, getting the initial probe alignment correct relative to the aircraft longitudinal axis or flight reference axis has been a problem, and if the bores for the dowel pins or the dowel pins themselves are not very precisely positioned so the probe is not properly aligned, the ability to accurately measure angle of attack, in particular, is impaired.
The present invention relates to an apparatus for aligning air data sensing probes properly in relation to the central longitudinal axis (or aircraft flight reference axis) of an aircraft in the pitch measuring plane. Angle of attack measurements determine the change in the angle of the longitudinal axis of the barrel of a probe (and thus the change in the angle of a reference longitudinal axis of the aircraft) relative to a flight path axis or plane. The probe is oriented at a selected angle relative to the central or horizontal axis of the aircraft. The position of the axis of the probe is selected to be consistent for repeatable angle of attack measurements.
In many instances, a zero angle of attack is where the probe axis actually is inclined relative to a flight path axis or plane of the aircraft. The angular difference between a horizontal reference plane when the longitudinal axis of the aircraft is horizontal and the axis of the barrel of the probe for correct angle of attack measurements is now determined by using wind tunnel tests or computational fluid dynamics. This difference angle is dependent upon the configuration of the aircraft. It can be different for different aircraft, but is repeatable between the same model of aircraft. This means that the correct orientation of the probe can be obtained by referencing the probe longitudinal axis to a horizontal plane, when the aircraft longitudinal axis is horizontal.
The present invention includes an alignment tool that mounts onto the base plate of a probe to be aligned and is held precisely positioned on the base plate with dowel pins. The probe is supported on an aircraft that has been previously leveled. Fasteners that pass through short, part annular slots on the probe base plate are avoided. The fasteners are adjusted to frictionally hold the probe from rotation under gravity but yet permit manually rotating the probe base for changing the angle of the longitudinal axis of the probe relative to the aircraft reference axis.
The alignment tool has a mounting plate that overlies at least a portion of the probe base plate and has an opening for clearance of the probe. The alignment tool mounting plate has dowel pins that fit into alignment bushings on the probe base plate for proper positioning of the tool, and the tool is secured so it can be rotated about a pitch axis with the probe base plate.
The probe base plate is guided on the mounting wall of the aircraft, as shown using thumb screws that pass through short slots on the probe base plate. Then the alignment tool and the probe are leveled with a leveling indicator. The leveling of the alignment tool is done by rotating the probe about the central axis of the probe base plate, which is normal to the pitch axis of the aircraft.
In one form shown, the alignment tool comprises an elongated rod mounted on the plate with circular disc members at the opposite ends of the rod. The rod protrudes in fore and aft direction both ahead of and behind the probe base plate, and suitable level indicator lines are provided on the circular disc members to provide markings or indicia that can be precisely oriented relative to a level laser beam or an optical line. Also, a tangent line on the lower surfaces of the end members is a level line. This level line is at a previously determined optimal angle relative to the axis of the probe, when the tool plate dowels are positioned in bushings on the probe base plate. The angle for the probe axis relative to a level plane can be found using wind tunnel tests or computational fluid dynamics and is aircraft dependent, as previously stated.
In a second form of the invention, a digital inclinometer is utilized for aligning the probe, and eliminates the need for a separate laser source and level spaced from the aircraft. The aircraft is still leveled, as an initial step, and then the alignment tool having a digital inclinometer mounted thereon is coupled to the air data sensor base plate, as in the first form of the invention. The alignment tool or fixture includes a tool mounting wall that is precisely positioned (with dowels) on the probe base plate. Dowel pins on the tool mounting wall fit into bores on the probe base plate for properly locating the tool as described before. The inclinometer is supported on an arm on the tool that permits orienting the inclinometer at different positions for providing an indication of xe2x80x9clevelxe2x80x9d when aligning probes requiring different angular positions, the alignment tool thus can be used for aligning several different sensors.
Once the alignment tool mounting wall is doweled into place on the probe base plate of the aircraft instrument, and the air data sensor base plate is held on the aircraft, but without securing it in place, the digital inclinometer is oriented on its support arm appropriately for the particular air data sensor being aligned, and the air data sensor is rotated about a horizontal or pitch axis until the digital inclinometer indicates that a level position or a desired, slightly off level position in the pitch axis direction has been achieved.
The air data sensor or probe base plate is tightened in place, and a cover plate is installed, and after flight tests the alignment of the probe is rechecked as explained before permanently securing the air data sensor in place.
In all forms of the invention, once the tool mounting plate and the probe base plate have been rotated (the tool mounting plate and base plate are doweled or guided together so they move as a unit), so that the alignment tool or inclinometer indicates the appropriate alignment angle for the probe of the air data sensor, the probe is temporarily secured in place for flight test and the fixture or tool is then removed. After flight test and rechecking proper alignment, if needed, precisely sized holes or bores are drilled into the aircraft mounting wall or mounting plate through the alignment bushings that are provided on the tool plate flange or base. Dowel pins are then pressed into the holes in the aircraft wall and extend into the alignment bushings of the sensor base plate. Thereafter, any air data sensing probe having an appropriate base plate that is installed at this location on the dowel pins in the aircraft wall will be aligned correctly. The probe base plate alignment bushings are preferably stainless steel inserts that have precise diameter bores that will fit onto the dowel pins.